Seven victims were killed Saturday night when a van drove into pedestrians on London Bridge before men jumped out and started attacking people with knives, according to witnesses and authorities.
Police shot three assailants dead in the attack, which also left 48 people injured.
Police carried out raids Sunday morning in east London and made 12 arrests, according to reports.
The attacks, which have been called "terrorist incidents" by the Metropolitan Police, began on London Bridge just after 10 p.m., when the nearby area was packed with revelers enjoying a Saturday night out.
First, a van veered off the road, knocking down pedestrians on the bridge.
According to witnesses and police, three men then jumped from the van with large knives and began attacking people at bars and restaurants at the popular Borough Market, which is less than a quarter of a mile south of the bridge.
"Everyone was running," a witness told Sky News.
Armed officers responded and shots were fired. Authorities said three attackers were killed but the investigation is continuing.
A BBC reporter, Holly Jones, was on the bridge when the van mowed down pedestrians. She said she believed the van was traveling at about 50 miles an hour.
"He swerved right round me and then hit about five or six people. He hit about two people in front of me and then three behind," Jones told the BBC.
After the attackers fled, Gerard Vowls told The Guardian that he saw a woman being stabbed by three men at the end of London Bridge. He threw chairs and bottles at the attackers to try to make them stop, he said.
"They kept coming to try to stab me. They were stabbing everyone. Evil, evil people," he said. "I want to know if this girl is still alive. I've been walking around for an hour and a half crying my eyes out. I don't know what to do."
Late Saturday night, British Prime Minister Theresa May said the incidents, which come nearly two weeks after the deadly attack in Manchester, are being treated as a "potential act of terrorism."
"This is a fast moving investigation," she said. "I want to express my huge gratitude to the police and emergency services who are on the scene. Our thoughts are with those who are caught up in these dreadful events."
In a speech outside 10 Downing Street on Sunday morning, she said there is "far too much tolerance" of extremism in the UK, and it's "time to say enough is enough."
"That will require some difficult and often embarrassing conversations, but the whole of our country needs to come together to take on this extremism we need to live our lives not in a series of segregated, separated communities, but as one truly United Kingdom," she said.
It is the third terror attack in the UK in recent months.
Just three months ago, a vehicle slammed into pedestrians on London's Westminster Bridge outside the Houses of Parliament. Five people, including a police officer, were killed and more than 50 others were injured in the terror attack on March 22.
On May 22, a suicide bomber detonated a device packed with nails at Manchester Arena following an Ariana Grande concert. In what was Britain's deadliest terror attack since July 2005, the blast left 22 victims dead, including children, and more than 100 injured.
On Saturday night, the leader of the Labour Party, Jeremy Corbyn, shared his thoughts on Twitter.
Brutal and shocking incidents reported in London. My thoughts are with the victims and their families. Thank you to the emergency services.— Jeremy Corbyn (@jeremycorbyn) June 3, 2017
Following the attack, President Donald Trump touted his travel ban.
We need to be smart, vigilant and tough. We need the courts to give us back our rights. We need the Travel Ban as an extra level of safety!— Donald J. Trump (@realDonaldTrump) June 3, 2017
Whatever the United States can do to help out in London and the U. K., we will be there - WE ARE WITH YOU. GOD BLESS!— Donald J. Trump (@realDonaldTrump) June 3, 2017